Methods for forming memory devices with reduced operational energy in phase change material

ABSTRACT

Methods of forming and operating phase change memory devices include adjusting an activation energy barrier between a metastable phase and a stable phase of a phase change material in a memory cell. In some embodiments, the activation energy barrier is adjusted by applying stress to the phase change material in the memory cell. Memory devices include a phase change memory cell and a material, structure, or device for applying stress to the phase change material in the memory cell. In some embodiments, a piezoelectric device may be used to apply stress to the phase change material. In additional embodiments, a material having a thermal expansion coefficient greater than that of the phase change material may be positioned to apply stress to the phase change material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/480,041, filed Jun. 8, 2009, now U.S. Pat. No. 8,031,518 issued Oct.4, 2011, the disclosure of which is hereby incorporated herein by thisreference in its entirety.

TECHNICAL FIELD

Embodiments of the present invention relate to methods, structures, anddevices for reducing the energy required to perform a writing operation,a reset operation, or both a writing operation and a reset operation inphase change memory devices.

BACKGROUND

Various types of non-volatile memory devices employ materials that canbe selectively caused to exhibit more than one value of electricalresistivity. To form a single memory cell (i.e., one bit), a volume ofsuch a material may be provided between two electrodes. A selectedvoltage (or current) may be applied between the electrodes, and theresulting electrical current (or voltage) therebetween will be at leastpartially a function of the particular value of the electricalresistivity exhibited by the material between the electrodes. A higherelectrical resistivity may be used to represent a “1” in binary code,and a lower electrical resistivity may be used to represent a “0” inbinary code, or vice versa. By selectively causing the material betweenthe electrodes to exhibit higher and lower values of electricalresistivity, the memory cell can be selectively characterized asexhibiting either a 1 or a 0 value.

One particular type of such non-volatile variable resistance memorydevices is the phase change memory device. In a phase change memorydevice, the materials provided between the electrodes typically arecapable of exhibiting at least two microstructural phases or states,each of which exhibits a different value of electrical resistivity. Forexample, the so-called “phase change material” may be capable ofexisting in a crystalline phase (i.e., the atoms of the material exhibitrelatively long range order) and an amorphous phase (i.e., the atoms ofthe material do not exhibit any or relatively little long range order).Typically, the amorphous phase is formed by heating at least a portionof the phase change material to a temperature above the melting pointthereof, and then allowing the phase change material to rapidly cool,which results in the material solidifying before the atoms thereof canassume any long range order. To transform the phase change material fromthe amorphous phase to a crystalline phase, the phase change material istypically heated to an elevated temperature below the melting point, butabove a crystallization temperature, for a time sufficient to allow theatoms of the material to assume the relatively long range orderassociated with the crystalline phase.

For example, Ge₂Sb₂Te₅ (often referred to as “GST”) is often used as aphase change material. This material has a melting point of about 620°C., and is capable of existing in amorphous and crystalline states. Toform the amorphous (high resistivity) phase, a portion of the materialis heated to a temperature above the melting point thereof by passing acurrent through the material between the electrodes and heating thematerial (the heat being generated due to the electrical resistance ofthe phase change material) for as little as 10 to 100 nanoseconds. Asthe GST material quickly cools when the current is interrupted, theatoms of the GST do not have sufficient time to form an orderedcrystalline state, and the amorphous phase of the GST material isformed. To form the crystalline (low resistivity) phase, a portion ofthe material may be heated to a temperature of about 550° C., which isabove the crystallization temperature and near, but below, the meltingpoint of the GST material, by passing a lower current (lower than thecurrent used in forming the amorphous phase, as described above) throughthe GST material between the electrodes to heat the GST material (to atemperature above the crystallization temperature but below the meltingpoint) for an amount of time (e.g., as little as about 30 nanoseconds)to allow the atoms of the GST material to assume the long range orderassociated with the crystalline phase, after which the current flowingthrough the material may be interrupted. One of the melting current (thecurrent passed through the phase change material to form the amorphousphase) and the crystallization current (the current passed through thephase change material to form the crystalline phase) may be referred toas the “write current,” and the other of the melting current and thecrystallization current may be referred to as the “reset current.” Thewrite current and the reset current may be collectively referred to asthe “programming currents.”

Various memory devices having memory cells comprising variableresistance material, as well as methods of forming and using such memorydevices are known in the art. For example, memory cells comprisingvariable resistance materials and methods of forming such memory cellsare disclosed in U.S. Pat. No. 6,150,253 to Doan et al. (issued Nov. 21,2000), U.S. Pat. No. 6,294,452 to Doan et al. (issued Sep. 25, 2001),United States Patent Application Publication No. 2006/0034116 A1 to Lamet al. (published Feb. 16, 2006), U.S. Pat. No. 7,057,923 to Furkay etal. (issued Jun. 6, 2006), United States Patent Application PublicationNo. 2006/0138393 A1 to Seo et al. (published Jun. 29, 2006), and UnitesStated Patent Application Publication No. 2006/0152186 A1 to Suh et al.(published Jul. 13, 2006). Furthermore, supporting circuitry that may beused to form a memory device comprising memory cells having a variableresistance material, as well as methods of operating such memorydevices, are disclosed in, for example, United States Patent ApplicationPublication No. 2005/0041464 A1 to Cho et al. (published Feb. 24, 2005),U.S. Pat. No. 7,050,328 to Khouri et al. (issued May 23, 2006), and U.S.Pat. No. 7,130,214 to Lee (issued Oct. 31, 2006).

The high amounts of energy required to heat a volume of phase changematerial for the programming operations (e.g., writing and resetoperations) in phase change memory devices has hindered their widespreadimplementation in the memory device market. Thus, there is a need in theart for methods, structures, and devices for decreasing the requiredprogramming energy (i.e., current) in phase change memory devices andsystems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating an example of anasymmetric quantum energy well for a hypothetical phase change materialcapable of existing in a stable phase and a metastable phase.

FIG. 2 is a cross-sectional view of a portion of an embodiment of amemory device of the present invention illustrating a single memory cellcomprising a phase change material and a piezoelectric material forstressing the phase change material.

FIG. 3 is a cross-sectional view of a portion of an embodiment of amemory device of the present invention illustrating a single memory cellcomprising a phase change material and a thermal expansion material forstressing the phase change material.

FIG. 4 is a cross-sectional view of a portion of an embodiment of amemory device of the present invention illustrating a single memory cellthat also comprises a phase change material and a thermal expansionmaterial for stressing the phase change material.

DETAILED DESCRIPTION

As used herein, the term “phase change material” means and includes anymaterial that is capable of existing in two or more (e.g., 2, 3, 4,etc.) solid phases (i.e., states) exhibiting at least one differentproperty or characteristic that can be detected, and that can be used inan electronic device to store information therein. Phase changematerials include, but are not limited to, chalcogenide materials suchas GeSbTe (GST), GeTe, AgInSbTe, InSe, SbSe, SbTe, InSbSe, InSbTe,GeSbSe, and GeSbTeSe.

As used herein, the term “piezoelectric material” means and includes anymaterial that will undergo mechanical deformation in the presence of anelectrical field. Piezoelectric materials have crystal structureswherein the unit cell of the crystal structure does not have a center ofsymmetry. By way of example, materials having perovskite ortungsten-bronze crystal structures exhibit piezoelectricity.Piezoelectric materials include, but are not limited to, quartz, galliumorthophosphate (GaPO₄), barium titanate (BaTiO₃), lead titanate(PbTiO₃), lead zirconate titanate (PZT) (Pb[Zr_(x)Ti_(1-x)]O₃, wherein0<x<1), and lithium tantalate (LiTaO₃).

As used herein, the term “thermal expansion material” means and includesany material that may be used to stress an adjacent phase changematerial in an electronic device or system by way of thermal expansionmismatch resulting from a difference between the thermal expansioncoefficient of the phase change material and the thermal expansioncoefficient of the thermal expansion material, and a change intemperature of the phase change material and the thermal expansionmaterial.

As used herein, the term “read operation” means an operation carried outwithin a memory device, in which operation a state of a memory cell isidentified without changing the state of the memory cell. For example,in a memory cell capable of existing in either of two states, onerepresenting a binary “0” and the other representing a binary “1,” aread operation would be used to determine whether the memory cell was inthe 0 state or the 1 state without changing the state of the memorycell.

As used herein, the term “write operation” means an operation carriedout within a memory device, in which operation a state of a memory cellis changed from a default state to a non-default state. For example, ina memory cell capable of existing in either of two states, onerepresenting a binary “0” and the other representing a binary “1,” the 0state may be designated as the default state, in which case the writeoperation would be used to change the state of the memory cell from thedefault 0 state to the non-default 1 state.

As used herein, the term “reset operation” means an operation carriedout within a memory device, in which operation a state of a memory cellis changed from a non-default state to a default state. For example, ina memory cell capable of existing in either of two states, onerepresenting a binary “0” and the other representing a binary “1,” the 0state may be designated as the default state, in which case the resetoperation would be used to change the state of the memory cell from thenon-default 1 state to the default 0 state.

For an improved understanding of embodiments of the invention, a briefdiscussion of general thermodynamic principles associated with changesbetween phases in a phase change material is set forth below withreference to FIG. 1. By way of example and not limitation, a volume ofsuch phase change material, capable of exhibiting in both a solid stablephase and another solid metastable phase, may be employed in embodimentsof memory devices of the present invention. The stable phase maycomprise a crystalline phase (i.e., a phase exhibiting long range atomicorder in crystal structure) of the phase change material, and themetastable phase may comprise an amorphous phase (i.e., a phase thatdoes not exhibit long range atomic order in crystal structure) of thephase change material.

FIG. 1 is a simplified schematic diagram illustrating an example of anasymmetric quantum energy well for a phase change material capable ofexisting in a stable phase and a metastable phase. In FIG. 1, the areaabove the curve represents available atomic energy states in the phasechange material. Although not shown in FIG. 1, a single energy state maybe theoretically represented by drawing a horizontal line across a spaceabove the curve. Lower energy states in the phase change material existat the lower regions of the area above the curve in FIG. 1. Absentexternal energy input, atoms within a material tend to fall toward andoccupy the lowest available atomic energy state. As shown in FIG. 1, theground state (the lowest energy state) in the stable phase is at arelatively lower energy level (point 10 at the bottom of the stablephase energy well) relative to the energy level of the ground state (thelowest energy state) in the metastable phase (point 12 at the bottom ofthe metastable energy well). The metastable phase of the phase changematerial is bound on the left side of the figure by an energy barrier 14and is bound on the right side of the figure by an energy barrier 16.The stable phase of the phase change material is bound on the right sideof the figure by an energy barrier 18 and is bound on the left side ofthe figure by an energy barrier 20.

As illustrated in FIG. 1, in order for the phase change material toswitch from the metastable phase to the stable phase, sufficient energymust be input into the phase change material to overcome the energybarrier 16, which is referred to herein as the activation energy E_(A).In other words, energy (e.g., heat) must be input into the atoms of themetastable phase material to raise their energy level to a point in FIG.1 above the energy barrier 16. The activation energy E_(A) is the energythat is required to move from the ground state of the metastable phaseinto the stable phase. As also illustrated in FIG. 1, in order for thephase change material to switch from the stable phase to the metastablephase, sufficient energy must be input into the phase change material toovercome the energy barrier 20. The energy barrier 20 is equal to thesum of the difference in energy ΔE between the ground states of themetastable phase and the stable phase and the activation energy E_(A).Thus, more energy must be input into the phase change material to switchthe phase change material from the stable phase to the metastable phasethan to switch the phase change material from the metastable phase tothe stable phase.

By way of example and not limitation, the stable phase of the phasechange material may comprise a crystalline phase exhibiting a relativelylower electrical resistance, and the metastable phase of the phasechange material may comprise an amorphous phase exhibiting a relativelyhigher electrical resistance. Thus, as a non-limiting example, a voltagemay be applied across the phase change material and the resultingcurrent may be measured to determine whether the phase change materialis in the crystalline stable phase or the metastable amorphous phase.

As known in the art, the phase change material may be selectivelyconverted back and forth between the crystalline stable phase and themetastable amorphous phase by passing electrical current through thephase change material (and, optionally, through an adjacent resistiveheating element) at a selected current magnitude and for a selectedamount of time. The flow of electrical current through the phase changematerial (and, optionally, through an adjacent resistive heatingelement) generates heat due to joule heating, and the temperature towhich the phase change material is heated is at least partially afunction of the amount of current flowing through the phase changematerial (and, more particularly, a function of the current densitywithin the phase change material). Thus, by controlling the magnitude ofthe current through the phase change material, the temperature to whichthe phase change material is heated may be controlled.

For example, to switch the phase change material from the crystallinestable phase to the amorphous metastable phase, a relatively higherelectrical current may be passed through the phase change material for arelatively short amount of time, after which the current may beinterrupted and the phase change material allowed to cool at arelatively rapid rate. During this process, sufficient energy is inputinto the phase change material to overcome the energy barrier 20illustrated in FIG. 1. To switch the phase change material from theamorphous metastable phase to the crystalline stable phase, a relativelylower electrical current may be passed through the phase change materialfor a relatively longer amount of time, after which the current may beinterrupted and the phase change material allowed to cool. During thisprocess, sufficient energy is input into the phase change material toovercome the energy barrier 16 (i.e., the activation energy E_(A))illustrated in FIG. 1. The particular values of current and time for anyparticular device will be at least partially a function of the materialcomposition of the phase change material, as well as the physicaldimensions of the phase change material in each memory cell of thedevice.

By way of example and not limitation, the crystalline stable phase maybe designated as a “1” in binary logic, and the amorphous metastablephase may be designated as a “0.” Furthermore, 0 may be arbitrarilydesignated as the default value of a memory cell in an electronic memorydevice. Thus, a write operation may be used to change a phase changematerial in a memory cell of an electronic memory device from anamorphous metastable phase to a crystalline stable phase, and, hence, towrite a 1 to the memory cell. A reset operation may be used to change aphase change material in a memory cell of an electronic memory devicefrom a crystalline stable phase to an amorphous metastable phase, and,hence, to reset the memory cell to 0. A read operation may be used todetermine whether a 1 or a 0 is stored in the memory cell (i.e., whetherthe phase change material is in the crystalline stable phase or theamorphous metastable phase).

In accordance with embodiments of the present invention, a phase changematerial may be physically stressed to reduce the activation energyE_(A) between phases of a phase change material, which may result in alower current required to perform a reset operation, a write operation,or both a reset operation and a write operation. In other words, a forceor forces may be applied to a phase change material in a memory cell ofa memory device, resulting in stress within the phase change material.The stress within the phase change material may reduce the energyrequired to convert the phase change material between at least twophases (e.g., a solid, amorphous metastable phase and a solid,crystalline stable phase, as described herein above) in which the phasechange material may exist.

Studies have been performed on the effect of pressure on thecrystallization of amorphous GeTe. See Wu, C. T., Luo, H. L., “PressureEffect on Vapor-Deposited Amorphous Materials,” Journal ofNon-Crystalline Solids 18, pp. 21-28 (1975). In particular, it has beenobserved that when a hydrostatic pressure was applied to amorphous GeTeat a rate of 500 MPa per minute to a maximum pressure of 1600 MPa, andfor a duration of four minutes, approximately 5.1% of the GeTe hadcrystallized. In other words, the percent crystallization X was 5.1%.When the maximum pressure was increased to 1850 MPa, approximately 11.6%of the GeTe had crystallized (i.e., X was 11.6), and approximately 16.1%of the GeTe crystallized (i.e., X was 16.1) when the maximum pressurewas further increased to 2000 MPA. Furthermore, it was observed that therate at which pressure was applied also affected the degree ofcrystallization in the GeTe. Thus, a relationship (which may beexpressed mathematically by an equation that would be different fordifferent material systems) may be established (e.g., heuristically)between the pressure applied to a phase change material and the degreeof crystallization that results after a given temperature and time attemperature. For the data given above, a plot of the pressure versus thepercent crystallization (X) may be generated, and a line may be fit tothe data points to generate a mathematical relationship therebetween.

A relationship also may be established (e.g., theoretically) between theactivation energy E_(A) between an amorphous phase and a crystallinephase of a phase change material and the degree of crystallization inthe material for a given temperature and time at temperature. Forexample, Johnson-Mehl-Avrami-Kolmogorov (JMAK) crystallization theorypredicts a relationship between the activation energy E_(A) ofcrystallization and the degree of crystallization for a giventemperature and time at temperature, which relationship is expressed byEquation 1 below:

$\begin{matrix}{{E_{A} = {{- {kT}}\;{\ln\left( {\frac{1}{k_{0}} \cdot \frac{1}{t - \tau} \cdot \left( {- {\ln\left( {1 - X} \right)}} \right)^{\frac{1}{n}}} \right)}}},} & {{Equation}\mspace{14mu} 1}\end{matrix}$wherein k is the Boltzmann constant, T is temperature, k₀ is the attemptfrequency factor, t is the time at temperature, i is the nucleationtime, X is the percent crystallization, and n is the Avrami exponent.The variables k₀, τ, and n in Equation 1 are material-specific variablesthat are known in the art for many phase change materials, or can bedetermined or estimated for any particular material using methods knownin the art.

Thus, using relationships like those discussed above, a relationshipbetween pressure (hydrostatic or axial) and the activation energy E_(A)between an amorphous phase and a crystalline phase (or between otherphases) of a phase change material may be identified, and the effect ofany given pressure on the activation energy E_(A) may be determined orestimated using that identified relationship. In other words, anidentified mathematical relationship (identified heuristically orotherwise) between the degree of crystallization (X) and the pressuremay be solved for the degree of crystallization (X) and substituted intoEquation 1 above to provide a relationship between applied pressure(and, hence, stress (σ) within the phase change material) and theactivation energy E_(A).

In some embodiments of the present invention, the phase change materialin memory cells of a memory device may be selectively stressed duringfabrication, and the selectively applied stress may be retained thereinupon completion of the fabrication process such that, when the memorydevice is subsequently used by an end user to store electronic datatherein, the activation energy E_(A) between phases of the phase changematerial is less than what it would be if the phase change material werein a relatively more relaxed state (e.g., an unstressed or a lessstressed state). For example, the packaging of the memory device may becarried out under conditions in which the phase change material isstressed, and, upon completion of the packaging process, the packagingof the memory device may serve to retain the stress within the phasechange material. In such embodiments, the selectively applied stresswithin the phase change material is a passive stress that is notintentionally changed during operation of the device.

As will be apparent to those of ordinary skill in the art, some degreeof stress may be generated in previously known phase change memorydevices during operation thereof due to mismatch in the coefficients ofthermal expansion of the various materials in previously known devices.As used herein, the term “selectively stressed” means intentionallystressed to a level beyond that which would be attained simply due tothermal expansion mismatch in previously known devices, although theexact magnitude of the stress level may in fact be undetermined. To theApplicant's knowledge, those of ordinary skill in the art haveheretofore sought to minimize such thermal expansion mismatch (and,hence, the stress resulting in the phase change material therefrom) toreduce structural damage that might occur as a result thereof. Thus,embodiments of the present invention are, surprisingly, structured andmay be caused to operate in a manner that is counterintuitive toconventional approaches in the relevant art.

In additional embodiments of the present invention, the phase changematerial in memory cells of a memory device may be actively selectivelystressed during operation of the device. In other words, the stresswithin the phase change material may change in an appreciable mannerduring operation of the device. By way of example and not limitation,the stress of the phase change material may be selectively subjected todifferent stress levels during operation of the device (e.g., subjectedto compressive stress, subjected to tensile stress, or sequentiallysubjected to both compressive and tensile stress).

In some embodiments, a piezoelectric material may be used to selectivelystress a phase change material in at least one memory cell of anembodiment of a memory device of the present invention. In other words,strain may be induced in a piezoelectric material disposed proximate aphase change material in a memory cell, and the strain of thepiezoelectric material may be used to apply force to the phase changematerial. The applied force generates stress within the phase changematerial, and the stress within the phase change material may result ina reduction in the activation energy E_(A), and, hence, the electricalcurrent, required to perform a write operation, a reset operation, orboth a write operation and a reset operation.

FIG. 2 is a cross-sectional view of a portion of an embodiment of amemory device 50 of the present invention. The memory device 50 includesan array of phase change memory cells, only one of which memory cells isillustrated in FIG. 2. Each phase change memory cell in the arrayincludes a first electrode 52 (e.g., which may comprise the electrodecommonly referred to in the art as the “bottom electrode” or “BEC”), asecond electrode 54 (e.g., which may comprise the electrode commonlyreferred to in the art as the “top electrode” or “TEC”), and a spatiallyconfined volume of phase change material 56 disposed between the firstelectrode 52 and the second electrode 54. In some embodiments, the firstelectrode 52 may comprise a conductive member 63 (e.g., a conductivetrace or pad) in electrical contact with a resistive heating element 64structured to generate heat as current flows through the resistiveheating element 64 (and the phase change material 56) between theconductive member 63 and the second electrode 54.

The memory device 50 further includes at least one piezoelectric devicethat includes a piezoelectric material 58. The piezoelectric device mayfurther include a field-generating device for providing an electricalfield within the piezoelectric material 58. For example, thefield-generating device may comprise a pair of electrodes 62A, 62Bbetween which a voltage may be provided. The voltage differentialbetween the pair of electrodes 62A, 62B may result in an electricalfield in a region encompassing at least a portion of the piezoelectricmaterial 58.

The piezoelectric material 58 may be located, and a crystallinestructure of the piezoelectric material 58 may be oriented, in such amanner as to cause the piezoelectric material 58 to apply stress to thephase change material 56 when an electrical field is provided within thepiezoelectric material 58 by the field-generating device (e.g., when avoltage is applied between the electrodes 62A, 62B). As a non-limitingexample, the crystalline structure of the piezoelectric material 58 maybe oriented such that, when an electrical field is provided within thepiezoelectric material 58 by applying a voltage between the electrodes62A, 62B, the piezoelectric material 58 expands or contracts in adirection oriented substantially parallel to a plane of the memorydevice 50 (i.e., the horizontal direction in the perspective of FIG. 2).The piezoelectric material 58 may also expand or contract in a directionoriented substantially perpendicular to the plane of the memory device50. Due to the Poisson effect, if the piezoelectric material 58 expandsin the horizontal direction in the perspective of FIG. 2, it will alsocontract in the vertical direction, and vice versa.

In some embodiments, a dielectric material 66 may be provided betweenthe phase change material 56 and the piezoelectric material 58 toelectrically insulate the phase change material 56 from thepiezoelectric material 58.

As shown in FIG. 2, the volume of phase change material 56 may comprisea first end surface 70 proximate (e.g., adjacent) the first electrode52, a second end surface 72 proximate (e.g., adjacent) the secondelectrode 54, and at least one lateral side surface 74 extending betweenthe first end surface 70 and the second end surface 72. As anon-limiting example, the volume of phase change material 56 may have arectangular (e.g., square) transverse cross-sectional shape, in whichcase, the volume of phase change material 56 will have four lateral sidesurfaces 74 extending between the first end surface 70 and the secondend surface 72. As another non-limiting example, the volume of phasechange material 56 may have a generally cylindrical shape (and, hence, agenerally circular transverse cross-sectional shape), in which case thevolume of phase change material 56 will have a single generallycylindrical lateral side surface 74 extending between the first endsurface 70 and the second end surface 72.

The piezoelectric material 58 may be disposed laterally beside the phasechange material 56. In some embodiments, the piezoelectric material 58may at least partially surround the lateral side surface or surfaces 74of the volume of phase change material 56. For example, thepiezoelectric material 58 may entirely laterally surround the volume ofphase change material 56. In other words, the piezoelectric material 58may extend entirely circumferentially around the phase change material56.

In additional embodiment of the present invention, the field-generatingdevice (e.g., the electrodes 62A, 62B) may be disposed remote from thepiezoelectric material 58. For example, the field-generating device maybe disposed remote from the piezoelectric material 58 within the memorydevice 50, or it may even be disposed outside the memory device 50(e.g., as a separate device used in conjunction with the memory device50).

When using the memory device 50, the field-generating device may be usedto selectively apply an electrical field within the piezoelectricmaterial 58 and cause the piezoelectric material 58 to expand and/orcontract in such a manner as to selectively apply stress to the phasechange material 56. Thus, for example, as the voltage, and the polarityof the voltage, between the electrodes 62A, 62B is selectivelycontrolled, the phase change material 56 may be selectively stressed,and, hence, the activation energy E_(A) between phases of the phasechange material 56 may be selectively adjusted (e.g., controlled).

The strain (∈) resulting in the piezoelectric material 58 and in thephase change material 56 may be used to estimate the magnitude of thestress a within the piezoelectric material 58 and in the phase changematerial 56 using the equation σ=E∈, wherein E is the elastic (Young's)modulus, ands ∈ is the strain in a body of material. Thus, the stress σwithin the phase change material 56 will be a function of a number ofvariables including the strain ∈ that can be generated in thepiezoelectric material 58 and the phase change material 56 by theapplied electrical field, and the elastic moduli E of the phase changematerial 56 and the piezoelectric material 58. Thus, the piezoelectricmaterial 58 may be used to selectively stress the phase change material56 and alter an activation energy E_(A) barrier between phases of thephase change material 56 in a desirable manner.

Methods other than those employing piezoelectric materials may be usedto selectively stress a phase change material in a memory cell to anextent sufficient to cause an appreciable reduction (e.g., a reductionof about 0.05 electron volt (eV) or more) in the activation energy E_(A)(FIG. 1) between phases of the phase change material in accordance withadditional embodiments of the invention. By way of example and notlimitation, principles of thermal expansion mismatch may be used toselectively stress a phase change material in a memory cell, asdescribed in further detail below.

FIG. 3 is a cross-sectional view of a portion of another embodiment of amemory device 80 of the present invention. The memory device 80 includesan array of phase change memory cells, only one of which is illustratedin FIG. 3. Each phase change memory cell in the array may besubstantially similar to the memory cell previously described withreference to FIG. 2, and may include a first electrode 52, a secondelectrode 54, and a spatially confined volume of phase change material56 disposed between the first electrode 52 and the second electrode 54.In some embodiments, the first electrode 52 may comprise a conductivemember 63 in electrical contact with a resistive heating element 64structured to generate heat as current flows through the resistiveheating element 64, as previously discussed.

The memory device 80 further includes a thermal expansion material 82proximate the phase change material 56. The thermal expansion material82 comprises a material that exhibits a coefficient of thermal expansionthat differs from a coefficient of thermal expansion exhibited by thephase change material 56. The thermal expansion material 82 is selectedto generate stress within the phase change material 56 upon heating orcooling of the thermal expansion material 82 and the phase changematerial 56 (e.g., during operation of the memory device 80), as aresult of the change in temperature and the difference in thermalexpansion coefficients (i.e., thermal expansion mismatch between thethermal expansion material 82 and the phase change material 56).

The thermal expansion material 82 may be disposed laterally beside thephase change material 56. In some embodiments, the thermal expansionmaterial 82 may at least partially surround the lateral side surface 74of the volume of phase change material 56. For example, the thermalexpansion material 82 may entirely laterally surround the volume ofphase change material 56. In other words, the thermal expansion material82 may extend entirely circumferentially around the phase changematerial 56.

In some embodiments, the thermal expansion mismatch between the thermalexpansion material 82 and the phase change material 56 may be sufficientto reduce a magnitude of the activation energy E_(A) between phases ofthe phase change material 56 by about 0.05 electron volt (eV) or more.

By way of example and not limitation, the thermal expansion material 82may exhibit a coefficient of thermal expansion that is at least aboutone and one-half (1.5) times a maximum coefficient of thermal expansionexhibited by the phase change material 56. As a non-limiting example,the amorphous phase of GeSbTe may exhibit a thermal expansioncoefficient of about 13.3 ppm/° K, and the crystalline phase of GeSbTemay exhibit a thermal expansion coefficient of about 17.4 ppm/° K. Thus,in embodiments in which the phase change material 56 comprises GeSbTe,the thermal expansion material 82 may exhibit a thermal expansioncoefficient of about 26.1 ppm/° K or more. As a non-limiting example,the thermal expansion material 82 may comprise benzocyclobutene, whichexhibits a thermal expansion coefficient of about 42 ppm/° K.

During operation of the memory device 80, electrical current flowsthrough the phase change material 56 from the first electrode 52 to thesecond electrode 54. The electrical resistance of the resistive heatingelement 64 generates thermal energy (i.e., heat), some of which isconducted into the phase change material 56 for switching phasesthereof. Some of this thermal energy, however, flows laterally outward(in the horizontal direction in the perspective of FIG. 3) away from theresistive heating element 64 and is not conducted into the phase changematerial 56. This portion of the thermal energy generated by theresistive heating element 64 is essentially wasted in prior art devices.In the memory device 80, however, at least some of this otherwise wastedthermal energy flows into the thermal expansion material 82. As thethermal expansion material 82 is heated by a portion of the thermalenergy generated by the resistive heating element 64, the thermalexpansion material 82 physically expands at a faster rate than does thephase change material 56. As a result, the phase change material 56 isselectively stressed as the thermal expansion material 82 expands aroundthe phase change material 56. This stress applied to the phase changematerial 56 may be sufficient to reduce a magnitude of the activationenergy E_(A) between phases of the phase change material 56 in anappreciable manner (e.g., by about 0.05 electron volt (eV) or more).

The strain (∈) resulting in the thermal expansion material 82 and in thephase change material 56 resulting from a given change in temperaturemay be determined using the equation ∈=αΔT, wherein α is the coefficientof thermal expansion and ΔT is the change in temperature. The magnitudeof the stress σ within the thermal expansion material 82 and in thephase change material 56 due to the strain ∈ may be estimated using theequation σ=E∈, wherein E is the elastic (Young's) modulus, and ∈ is thestrain in a body of material. Thus, the stress σ within the phase changematerial 56 will be a function of a number of variables including thecoefficients of thermal expansion α of the phase change material 56 andthe thermal expansion material 82, as well as the elastic moduli E ofthe phase change material 56 and the thermal expansion material 82.Thus, the particular material compositions of the phase change material56 and the thermal expansion material 82 may be selected to increase(e.g., maximize) the stress within the phase change material 56 relativeto that in conventional devices, or otherwise alter an activation energyE_(A) barrier between phases of the phase change material 56 in adesirable manner.

FIG. 4 is a cross-sectional view of a portion of yet another embodimentof a memory device 90 of the present invention. The memory device 90 ofFIG. 4 is substantially similar to the memory device 80 of FIG. 3. Thememory device 90, however, includes a thermal expansion material 92proximate the phase change material 56 that also extends laterallybeside (e.g., laterally adjacent) at least a portion of the firstelectrode 52 (e.g., laterally adjacent at least a portion of theresistive heating element 64). In this configuration, relatively morethermal energy may flow from the first electrode 52 into the thermalexpansion material 92 to further increase the thermal expansion of thethermal expansion material 92.

It is noted that embodiments of the present invention may be used toselectively stress phase change material 56 in a phase change memorycell. The programming and reset current of a phase change memory cellcan also be reduced by controlling the resistivity of the phase changematerial 56. In conventional phase change materials, the resistance ofthe phase change material 56 drops off significantly as the temperaturerises. Once the resistance of the phase change material 56 is reduced,the joule heating of the phase change material 56 resulting from theresistivity of the phase change material 56 also is reduced. It is forthis reason that the first electrode 52 (the bottom electrode (BEC)) inconventional phase change memory cells includes a resistive heatingelement 64, the resistance in which provides the joule heating thatcontinues to heat the phase change material 56 after the resistance ofthe phase change material 56 is decreased. This, however, is arelatively inefficient approach, since the heating of the phase changematerial 56 is indirectly provided by the resistive heating element 64,and much of the heat generated thereby does not flow into the phasechange material 56, as previously discussed. Thus, it would beadvantageous to maintain the resistivity of the phase change material 56at levels high enough to at least primarily (e.g., entirely) heat thephase change material 56 directly by the resistive joule heating of thephase change material 56 itself.

The resistivity of the phase change material 56 may be modulated withmechanical stress. Thus, embodiments of the present invention, which maybe used to selectively stress the phase change material in phase changememory cells, also may be used to selectively modulate the resistance ofthe phase change material 56. Consequently, in accordance with someembodiments of the present invention, the phase change material 56 maybe selectively stressed to maintain a resistance in the phase changematerial 56 above a threshold level during write operations and/or resetoperations, to directly heat the phase change material 56 at leastprimarily by joule heating within the phase change material 56, whichmay improve the power efficiency of embodiments of memory devices of thepresent invention relative to previously known memory devices. Forexample, in some embodiments, the phase change material 56 may be placedunder tensile stress to raise a resistance of the phase change material56 while the phase change material 56 is directly heated by passingcurrent therethrough, after which the phase change material 56 then maybe placed under compressive stress to reduce an activation energy E_(A)barrier between phases of the phase change material 56. Otheroperational schemes also may be advantageously employed in additionalembodiments of the invention.

In view of the above description, some embodiments of the presentinvention include methods of operating a memory device in which one of aread operation, a write operation, and a reset operation is performed ona memory cell of the memory device while a magnitude of an activationenergy barrier between a stable phase and a metastable phase of a phasechange material of the memory cell is at a first level, the magnitude ofthe activation energy barrier is selectively changed from the firstlevel to a second, different level, and another of the read operation,the write operation, and the reset operation is performed while theactivation energy barrier is at the second level.

Additional embodiments of the present invention include methods offabricating memory devices in which a phase change material in at leastone memory cell is selectively stressed to alter a magnitude of anactivation energy barrier between different phases of the phase changematerial, and the stress is retained in the phase change material atleast until fabrication of the memory device is complete.

Further embodiments of the present invention include memory devices thatinclude a phase change memory cell and a material, structure, or devicefor selectively stressing phase change material in the memory cell. Forexample, a memory device may include a piezoelectric device having apiezoelectric material located, and a crystalline structure of thepiezoelectric material oriented, to cause the piezoelectric material toapply stress to the phase change material when an electrical field isprovided within the piezoelectric material. The piezoelectric device mayalso comprise a field-generating device for providing an electricalfield within the piezoelectric material. As another example, a memorydevice may include a thermal expansion material at least partiallysurrounding the phase change material in a phase change memory cell. Thethermal expansion material has a coefficient of thermal expansiondiffering from a coefficient of thermal expansion of the phase changematerial. The thermal expansion material may be selected to generatestress within the phase change material upon heating or cooling of thethermal expansion material and the phase change material duringoperation of the memory device.

Additional embodiments of the present invention include methods ofoperating a phase change memory device in which a phase change materialin at least one phase change memory cell is selectively stressed tomaintain a resistance of the phase change material above a thresholdlevel, and current is passed through the phase change material whilemaintaining the resistance of the phase change material above thethreshold level to directly heat the phase change material at leastprimarily by joule heating within the phase change material.

While the present invention has been described in terms of certainillustrated embodiments and variations thereof, it will be understoodand appreciated by those of ordinary skill in the art that the inventionis not so limited. Rather, additions, deletions and modifications to theillustrated embodiments may be effected without departing from scope ofthe invention as defined by the claims that follow, and their legalequivalents.

What is claimed is:
 1. A method of fabricating a memory device,comprising: selectively stressing a phase change material in at leastone memory cell of a partially fabricated semiconductor device to reducea magnitude of an activation energy barrier between different phases ofthe phase change material, comprising: inducing strain in apiezoelectric material using an electric field to expand or contract thepiezoelectric material in a direction; and using the induced strain inthe piezoelectric material to stress the phase change material;completing fabrication of the memory device; and retaining the stress inthe phase change material at least until fabrication of the memorydevice is complete.
 2. The method of claim 1, wherein selectivelystressing the phase change material comprises stressing the phase changematerial to a stress of about 100 MPa or more.
 3. The method of claim 1,wherein selectively stressing the phase change material comprisesreducing the magnitude of the activation energy barrier by about 0.05 eVor more.
 4. The method of claim 1, further comprising selecting thephase change material to comprise GeSbTe.
 5. The method of claim 1,wherein selectively stressing the phase change material comprisesreducing the magnitude of the activation energy barrier.
 6. The methodof claim 1, further comprising selecting the piezoelectric material fromthe group consisting of quartz, gallium orthophosphate (GaPO₄), bariumtitanate (BaTiO₃), lead titanate (PbTiO₃), lead zirconate titanate (PZT)(Pb[Zr_(x)Ti_(1-x)]O₃, wherein 0<x<1), and lithium tantalate (LiTaO₃).7. The method of claim 1, further comprising disposing the piezoelectricmaterial laterally beside the phase change material.
 8. The method ofclaim 7, wherein the piezoelectric material has a crystal structureoriented such that, when an electrical field is provided within thepiezoelectric material, the piezoelectric material expands or contractsin the lateral direction.
 9. The method of claim 1, further comprisingproviding a dielectric material between the piezoelectric material andthe phase change material, the dielectric material physically isolatingthe piezoelectric material from the phase change material.
 10. A methodof fabricating a memory device, comprising: selectively stressing aphase change material in at least one memory cell of a partiallyfabricated semiconductor device to reduce a magnitude of an activationenergy barrier between different phases of the phase change material,the selectively stressing comprising: selecting a thermal expansionmaterial comprising benzocyclobutene; inducing strain in the thermalexpansion material by at least one of heating and cooling the thermalexpansion material; and using the induced strain in the thermalexpansion material to stress the phase change material; completingfabrication of the memory device; and retaining the stress in the phasechange material at least until fabrication of the memory device iscomplete.
 11. The method of claim 10, further comprising selecting thethermal expansion material to exhibit a coefficient of thermal expansionthat is at least about one and one-half (1.5) times a maximumcoefficient of thermal expansion exhibited by the phase change material.12. The method of claim 10, further comprising disposing the thermalexpansion material laterally adjacent the phase change material.